“Deserved” it? No… but it’s extremely easy to see why his killers thought he did.
Make peace with MUTANTS? PREPOSTEROUS!!
Apparently they aren’t thinking that clearly, because Matheson portrays them as being competely terrified of Neville. My read on that (and now I’m digging into my memory, but guess what book I’m going to re-read this week?) was that they thought he was murdering vampires regardless of type, and doing so with superhuman prowess. The final scenes of the book (with the scene at the window, and the last line) suggest he’s like an evil demi-god or demon in their eyes…they see him with atavistic terror.
As an aside, are the semi-vampires still citizens of the US? I mean, is the original rule of law considered gone and replaced with something else? I’d guess so. A book written from their viewpoint would be interesting.